THE MOLECULAR-GENETICS OF HEXOSE-TRANSPORT IN YEASTS

Transport across the plasma membrane is the first, obligatory step of hexose utilization. In yeast cells the uptake of hexoses is mediated b y a large family of related transporter proteins. In baker's yeast Sac charomyces cerevisiae the genes of 20 different hexose transporter-rel ated proteins have been identified. Six of these transmembrane protein s mediate the metabolically relevant uptake of glucose, fructose and m annose for growth, two others catalyze the transport of only small amo unts of these sugars, one protein is a galactose transporter but also able to transport glucose, two transporters act as glucose sensors, tw o others are involved in the pleiotropic drug resistance process, and the functions of the remaining hexose transporter-related proteins are not yet known. The catabolic hexose transporters exhibit different af finities for their substrates, and expression of their corresponding g enes is controlled by the glucose sensors according to the availabilit y of carbon sources. In contrast, milk yeast Kluyveromyces lactis cont ains only a few different hexose transporters. Genes of other monosacc haride transporter-related proteins have been found in fission yeast S chizosaccharomyces pombe and in the xylose-fermenting yeast Pichia sti pitis. However, the molecular genetics of hexose transport in many oth er yeasts remains to be established. The further characterization of t his multigene family of hexose transporters should help to elucidate t he role of transport in yeast sugar metabolism.